1. Field of the Invention
The present invention relates to a power supply circuit for driving a liquid crystal display unit, and more particularly to a power supply circuit for supplying a voltage at a common side.
2. Description of the Related Art
As a conventional power supply circuit for driving a liquid crystal display unit, there is known a power supply circuit employing a switching regulator as shown in a circuit diagram of FIG. 4. That is, there has been obtained positive and negative power supply voltages necessary for use as a drive voltage at a common side using a switching regulator and a transformer.
In a liquid crystal display unit using a drive method such as multi-line addressing (MLA), a voltage relationship of VH, VL and V1 which are drive voltages on the common side satisfies the relation VH-V1=V1-VL, as shown in FIG. 3. When this relationship is not satisfied, a residual image is produced on a display screen, thereby causing deterioration of display quality. However, in the conventional power supply circuit, because the accuracy in winding ratio of the transformer is difficult to obtain, there arises a problem that it is difficult to satisfy the above equality.